Degenerative disc disease is a common condition of the intervertebral disc of the spine characterized by disc height collapse with or without disc herniation, osteophyte formation, foramenal stenosis, facet hypertrophy, synovial cyst, and other symptoms. Any or a combination of these findings can lead to pain or neurological deficit. Many of the symptoms of degenerative disc disease may be alleviated by decompression of the neural structures and immobilization of the involved spinal segments. Immobilization is typically achieved in the long term by removal of the disc and placement of bone graft. Temporary immobilization to encourage incorporation of the bone graft can be achieved with placement of rigid hardware such as screws and rods.
While immobilization and a successful fusion may relieve the pain associated with nerve impingement, the long-term consequences of eliminating the motion of the intervertebral disc show a tendency toward increased risk of failure of the adjacent discs. The lack of motion at the fusion site places increased biomechanical demands on the adjacent discs causing them to degenerate prematurely.
Replacement prostheses have been suggested for degenerative disc disease to allow motion at the operative disc level. Several types of artificial intervertebral discs for replacing a part or all of a removed disc have been developed, such as, ball and socket discs, and mechanical spring discs. However, these devices are devoid of stiffness and stability and rely on the remaining spinal elements, such as the ligaments, muscles and remaining intervertebral disc tissue, namely the annulus fibrosis, for stability. For example, U.S. Pat. No. 5,556,431 to Buttner-Janz, U.S. Pat. No. 5,507,846 to Bullivant and U.S. Pat. No. 5,888,226 to Rogozinski, all of which are incorporated herein by reference, disclosed prostheses that comprise ball and socket type joints. The ball and socket disc prostheses typically incorporate two plate members having cooperating inner ball and socket portions that permit articulating motion of the members during movement of the spine. These inventions rely on stretching the annulus fibrosis to put the prosthesis into compression to gain stiffness. There are a risk of altering the spine's biomechanics by increasing the disc height past the normal range and/or a risk of damage to the annulus fibrosis. If the disc space is not stretched enough an unstable spinal segment could result, possibly leading to pain and further injury. In addition, this low stiffness places detrimentally high loads on supporting ligaments and muscles, particularly during movement involving torsional rotation of the spine. Dislocation and wear are other concerns with this disc type. Implantation entails insertion of several separate pieces that must be properly aligned during surgery. The surgery is often performed with a minimal incision offering limited access to the insertion site. Perfect alignment after insertion could be difficult.
Mechanical spring discs usually incorporate one or more coiled springs disposed between metal endplates. The coiled springs preferably define a cumulative spring constant sufficient to maintain the spaced arrangement of the adjacent vertebrae and to allow normal movement of the vertebrae during flexion and extension of the spring in any direction. Disadvantages of the mechanical spring disc types include attachment of the coiled springs to the metal end plates and associated wear at the attachment points. Examples of mechanical spring discs are disclosed in U.S. Pat. No. 5,458,642 to Beer et al. and U.S. Pat. No. 4,309,777 to Patil.
Other prostheses have been suggested, for examples, see U.S. Pat. Nos. 6,136,031 and 6,296,664 to Middleton, U.S. Pat. No. 5,320,644 to Baumgartner, U.S. Pat. No. 5,827,328 to Buttermann and U.S. Pat. No. 5,676,702 to Ratron, all of which are incorporated herein by reference. These disc prostheses have their own inherent stiffness, but may not take into account that axial loads placed on the spine during activity are generally much larger than bending loads. Therefore, these prostheses would either bottom out under axial loads and offer no response to bending loads, or be stiff enough to support the axial loads and thereby too stiff to flex under bending loads.
Therefore, a heretofore unaddressed need exists in the art to address the aforementioned deficiencies and inadequacies.